The present invention relates generally to compound semiconductor field effect transistors (FETs), and, more particularly, to a MESFET having an epitaxial void.
MESFETs have been developed in order to overcome the inherent limitations of metal oxide FETs (MOSFETs) fabricated in silicon chips, particularly with respect to operating speed. Because GaAs and other semi-insulating or compound semiconductor materials have a much higher electron mobility than silicon, MESFETs are capable of much higher operating speeds than MOSFETs.
With reference now to FIG. 1, a conventional delta-doped MESFET (.delta.-MESFET) will now be described. The conventional .delta.-MESFET includes a semi-insulating GaAs substrate 10, an undoped or p-type GaAs buffer layer 12, a delta-doped Si channel monolayer 14, and an undoped GaAs spacer layer 16, n.sup.+ type source and drain regions 18 formed in the surface of the p-type buffer layer 12, source and drain electrodes, 15, 17, formed on the source and drain regions 18, and a gate electrode 13 formed in Schottky contact with the spacer layer 16 between the source and drain electrodes 15, 17.
A gate voltage applied to the gate electrode 13 and bias voltages applied to the source and drain electrodes 15, 17 govern the thickness of a depletion region formed in the channel layer 14, and thus the current flowing through the channel of the MESFET between the source and drain thereof.
In the fabrication of the conventional MESFET, the gate electrode 13 is formed by means of a conventional photolithographic process. Consequently, conventional MESFETs often have misaligned gates and unduly high channel resistances. Further, because the buffer layer 12 is formed to suppress leakage current, the impurity concentration thereof is low, (e.g., 10.sup.14 ions/cm..sup.2), thereby further increasing the channel resistance and threshold voltage of the conventional MESFET. Moreover, because of the difficulties inherent in the process for fabricating the conventional MESFET, the buffer layer 12 is not effective to eliminate the backgating effect due to the voltage difference between the substrate and an electrode of a neighboring transistor, which problem is exacerbated when the electric field in the channel layer increases and/or the voltage applied to the neighboring electrode increases.
Based on the foregoing, it can be appreciated that there presently exists a need in the art for a MESFET which overcomes the above-described drawbacks and shortcomings of the conventional MESFETs. The present invention fulfills this need.